The present invention relates to devices for holding semiconductor wafers and similar articles in processing apparatus.
Many types of industrial processing operations require automated transfer of articles undergoing processing between successive processing stations. An exemplary operation of this type is semiconductor wafer fabrication, in which an article in the form of a semiconductor wafer is transferred between various processing units, each unit performing a different operation. Such operations include film deposition, ion implantation, impurity diffusion, etching, etc. which are performed in a processing chamber containing a suitable atmosphere.
Each processing unit includes some type of support for securely holding the wafer in position for processing. Known supports include those that employ mechanical clamps, as disclosed in U.S. Pat. No. 4,603,466 (M. J. Morley), and those which employ electrostatic forces, as disclosed in U.S. Pat. No. 5,103,367 (Horwitz et al) as well as in patents and publications cited therein.
When articles such as semiconductor wafers are transferred from one unit to another by automated equipment, there is always the possibility of a malfunction that results in a failure to deliver an article to a support or incorrect positioning of the article on the support. Therefore, in installations of this type, it is desirable to monitor correct positioning of a wafer before the start of a processing operation.
U.S. Pat. No. 5,103,367, cited above, discloses an electrostatic chuck in which voltages are applied to electrodes to produce electrostatic attraction forces. The voltages produce a current between each electrode and a reference electrode and the amplitude of the currents increases as a wafer is brought closer to the chuck. When the wafer is sufficiently close, these currents attain an amplitude which produces a switching effect to increase the voltage applied to the electrodes, thereby substantially increasing the electrostatic attraction forces.
Monitoring of wafer proximity according to this patent appears to be only for the purpose of determining when to apply an increased holding force, and it appears to be assumed that the wafer will be automatically placed in the correct position. There does not appear to be a provision for any alarm or other fault response if the wafer does not arrive at the correct position. Moreover, there does not appear to be any indication in this patent that the proximity sensing system could provide an indication if the wafer is on the chuck but is misaligned.
If a wafer that has been placed on the surface of the chuck is somewhat warped, or bowed, then the electrostatic attraction force which must be applied to place the wafer flat against the upper surface of the chuck generally will require a higher value than that which would be required for a perfectly flat wafer. It is therefore known to produce electrostatic attraction forces which are higher than those required by a perfectly flat wafer. However, such increased electrostatic attraction forces can cause the lower surface of the wafer to become scratched, and thereby possibly produce particles which might circulate in the processing chamber and contaminate the upper surface of the wafer.
Certain electrostatic chucks which are known in the art include passages via which gas is supplied to the underside of a wafer mounted on the chuck in order to heat the wafer to processing temperature. The pressure of gas flowing through the passages may be monitored in order to determine whether a wafer is present on the chuck. Exemplary chucks of this type are marketed by Applied Materials, Inc. of Santa Clara, Calif. under the product designation PVD-ECHUCK.
Systems of this type can very reliably indicate situations in which a wafer is completely absent. However, these systems are not intended to be able to indicate that a wafer while present on the chuck is nonetheless incorrectly positioned, or has a piece broken off, or is warped so that only a portion of the wafer contacts the wafer support surface of the chuck.
Another approach is to use capacitance to detect wafer position. For example, U.S. Pat. No. 5,436,790 issued Jul. 25, 1995 describes a wafer presence and clamp condition monitoring apparatus which monitors capacitance between two electrodes embedded within a wafer support pedestal. The capacitance falls into one range with no wafer positioned upon the support surface and into a second range with a wafer in place but not clamped. Furthermore, the capacitance falls in a third range with the wafer held in place by an electrostatic chuck formed when the embedded pair of electrodes are energized with a DC voltage. The monitoring circuit senses when the capacitance of the system is in each of the ranges by converting the measured capacitance to a DC voltage that can easily be sensed and used to confirm wafer placement and clamping.
Specifically, the electrostatic chuck used in the prior art system contains a pliable surface such that when the clamping force is applied, the wafer compresses the surface material and the wafer physically moves nearer to the pedestal surface and its embedded electrodes. This physical movement of the wafer relative to the electrodes causes a change in the capacitance between the electrodes. Such pliable surface materials are typically useful primarily in low temperature semiconductor processing systems. At high temperatures, these materials tend to breakdown, outgas and/or deform. Thus, at high temperatures, an electrostatic chuck having a pliable surface may contaminate the chamber.
Ceramic electrostatic chucks that are typically used in high temperature semiconductor wafer processing are constructed of a ceramic material that becomes somewhat conductive at high temperatures (i.e., the resistivity of the material decreases with increased temperature). Specifically, when the wafer rests flush against the surface of the chuck body while chucking voltage is applied to one or more embedded electrodes, the wafer is primarily retained against the ceramic support by the Johnsen-Rahbek effect. One example of such a ceramic chuck is disclosed in U.S. Pat. No. 5,117,121 issued May 26, 1992 and incorporated herein by reference.
Ceramic chucks of this type have a hard, non-pliable surface that generally does not breakdown or deform during high temperature wafer processing.
Copending application entitled "Method and Apparatus for Wafer Detection" Ser. No. 08/873,260, filed Jun. 11, 1997, (Attorney Docket 1913) is directed to a capacitive position sensor for such high temperature chucks. However, capacitive sensors have a number of drawbacks. For example, capacitive sensors typically require that at least one electrical lead be brought into the interior of the processing chamber. Since the interior of the chamber must be isolated from the region surrounding the chamber, the need to provide a feedthrough for that lead presents additional design and fabrication complications. In addition, known capacitive sensors can sense the pressure of a wafer, but typically cannot reliably indicate if the wafer is correctly positioned.
Therefore, a need exists in the art for other systems that detect wafer presence and operate at high wafer processing temperatures.